patch 8.0.0698: crash on exit when using Python function in timer.

Problem:    When a timer uses ":pyeval" or another Python command and it
            happens to be triggered while exiting a Crash may happen.
            (Ricky Zhou)
Solution:   Avoid running a Python command after python_end() was called.
            Do not trigger timers while exiting.  (closes #1824)

src/ex_cmds2.c

@@ -1183,6 +1183,7 @@ timer_callback(timer_T *timer)
 /*
  * Call timers that are due.
  * Return the time in msec until the next timer is due.
+ * Returns -1 if there are no pending timers.
  */
     long
 check_due_timer(void)
@@ -1197,6 +1198,10 @@ check_due_timer(void)
 # ifdef WIN3264
     LARGE_INTEGER   fr;
 
+    /* Don't run any timers while exiting. */
+    if (exiting)
+	return next_due;
+
     QueryPerformanceFrequency(&fr);
 # endif
     profile_start(&now);